Automatic power controls for amplifiers



y 1969 J. F. LALANDE ETAL 3,458,826

AUTOMATIC POWER CONTROLS FOR AMPLIFIERS Filed May 23, 1967 2Sheets-Sheet 1 FIG/I 7 AMP .F .=,L REFLET LOAD I g METOER S mpsomceGENERATOR w P P WATT E +0 L L METER SOURCE 16 l p I P "2 THRESHOLD 4ADDER /COMPARATOR RELAY p B 19 I We 3 July 29, 1969 J. F. LALANDE ETAL3,458,826

AUTOMATIC POWER CONTROLS FOR AMPLiFIERS Filed May 23, 1967 2Sheets-Sheet 2 FIG .2

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050. 27 22a 22' jOOULATOR VARIABLE GAIN 24 25 26 PRE-AMP I w 2:3/ MPcmcun METER 28 )fi IMF 4 JV0 0.0. H SOURCE United States Patent3,458,826 AUTOMATIC POWER CONTROLS FOR AMPLIFIERS Jean F. lLalande,Bagneux, and Alfred Sicliel, Paris, France, assignors to C.I.T.CompagnieIndustrielle des Telecommunications, Paris, France, a corporation ofFrance Filed May 23, 1967, Ser. No. 640,572 Int. Cl. H03f 1/30 U.S. Cl.330-2 Claims ABSTRACT OF THE DISCLOSURE A system for controlling theenergy dissipation in an amplifier connected between a signal source anda load by calculating the energy dissipation from the value of energysupplied by said amplifier to said load, the value of energy reflectedfrom said load back to said amplifier and the value of energy suppliedby the voltage source to the amplifier, and reducing the excitationlevel of said amplifier when the detected dissipation thereof exceeds amaximum value.

BACKGROUND OF THE INVENTION The invention relates to a regulating devicefor limiting automatically the D.C. energy absorbed by a power amplifierfor radio-electric waves, and more particularly a radio transmissionamplifier, by preventing excessive dissipation from occurring therein.

If the impedance match between an amplifier and a load connected by aline thereto is not perfect, the load will reflect a portion of theenergy travelling along the line from the amplifier to the load backtowards the amplifier. It is known that under these conditions theenergy P eifectively transmitted by the load is equal to the energy Ptransmitted by the amplifier towards the load, less the energy Preflected by the load back towards the amplifier, according to theequation:

It is also known that a part P of the power supplied by a source P tothe amplifier is dissipated in the amplifier, the rest of the powerbeing the energy P absorbed by the load according to the equation:

P4=P0 P1 or, according to Equation 3, obtained by combining Equations 1and 2:

It is important that the energy P dissipated by the amplifier should notexceed the safety value W, fixed by the designer.

In a typical case, where the output circuit is suitably matched andpresents to the amplifier its optimum load impedance, the dissipatedenergy is smaller than the safety limit, due to the control of variousparameters (supply voltage, polarity, excitation, and the like).However, in the case where, for some reason, the load impedance assumesa value different than the normal value, for example, in the case of amobile transmission amplifier, the change in the impedance of theantenna due to obstacles in the proximity, causes the reflection of theenergy to rise and there may thus occur an increase in the dissipatedenergy P, which could exceed the permitted value. In order to protectthe amplifier under these conditions, it is known to provide safetycircuits which most frequently act so as to simply cut off the supplysource, which causes an interruption of the transmission of the message.Such an interruption may be highly undesirable.

Known safety devices use generally the standing wave ratio as a measurein order to evaluate the mismatching 3,458,826 Patented July 29, 1969"ice BRIEF DESCRIPTION OF THE INVENTION The invention has the primaryobject of acting only in the case of danger to the amplifier to reducewithout completely cutting-01f the supply energy absorbed by thetransmitter so as to reduce the dissipated energy below the safety levelW. This is accomplished in accordance with the invention by adjustingautomatically the value of the absorbed power to the maximum permissiblevalue under all states applicable, that is to say, by observing thecondition P =W, or P W, or, more generally, P W.

Although the invention relates to power amplifiers generally, itconcerns more particularly transmitters equipped with a power amplifierof the type Class AB or Class B or Class C, whose dissipation varies asa function of the level of excitation. This facilitates the executionconsiderably, because it is suflicient to lower the level of excitationof this amplifier by low-level power means, which are therefore cheap,in order to obtain the control required with regard to the energydissipated in the amplifier.

For measuring the energy P transmitted by the amplifier to the load andthe power P reflected from the load towards the amplifier, the inventionuses an apparatus known per se, a so-called reflectometer. An apparatusof this kind, inserted into a transmission line, comprises a device formeasuring a voltage between conductors, a first current-responsivecoupling loop having a mutual inductance M with respect to the lineproviding the first current measurement, and a second loop having aninductance M with respect to the line providing a second currentmeasurement. By combining the currents supplied by the threeabove-mentioned measuring devices, one obtains at a first outputterminal a signal P proportional to the power transmitted by thegenerator to the load, and at a second output terminal a signalproportional to the power P reflected by the load towards the generator.

According to the invention, an automatic control device for controllingthe energy dissipated in a power amplifier supplied by a D.C. source andadapted to maintain the energy dissipated in the amplifier below amaximum value, comprises a calculating member having one output andthree inputs, two inputs of which are connected to the output terminalsof a reflectometer mounted between the said amplifier and its load, thethird input being connected to a wattmeter circuit measuring the energysupplied by the D.C. source, wherein a threshold device has a firstinput connected to the said calculating device and a second inputconnected to a predetermined D.C. voltage, and a control member for thedissipated energy of the amplifier, whose input is connected to theoutput of the said threshold device. The control member may act on thealternative excitation level of the power amplifier.

In the case where the D.C. source supplies a D.C. voltage having astabilized output, the wattmeter circuit mentioned above may be replacedby an ammeter circuit, formed by a low-value resistance mounted in theground return of the said D.C. source, whose end remote from ground isconnected to the said third input of the calculating member.

According to yet another feature of the invention, the singlecalculating member is replaced by two differential amplifiers incascade, a first differential amplifier receiving at its inputs thesignals coming from the reflectometer and a second diiferentialamplifier receiving at its inputs the output signal of the said firstamplifier and the abovementioned wattmeter signal.

In the case where the device according to the invention comprises asingle calculating member of the type known in the art as an adderrelay, the calculation made conforms to the Equation 3 above. In thecase where the device according to the invention is equipped with twodifferential amplifiers in cascade, the first amplifier carries out acalculation according to Equation 1, and the second differentialamplifier a calculation according to Equation 2.

The invention will be further described with reference to theaccompanying drawing, in which:

FIGURE 1 is a block diagram, showing the arrangement according to theinvention in its most general form;

FIGURE 2 is a more detailed diagram of a particular embodiment.

DETAILED DESCRIPTION OF THE INVENTION In FIGURE 1, an alternatinggenerator 11 provides electronic control to an amplifier 13 via avariable reducer 12, for example, a diode reducer. The device 12 can bean amplifier with variable gain excitation by means of electroniccontrol or other similar devices providing the required control. Thepower amplifier 13 acts on a load impedance 15 through a refiectometer14 with two output terminals which supply respectively, a signal Pvarying as the energy supplied by the amplifier to the load P and asignal P varying as the energy P reflected towards the amplifier by theload.

The amplifier 13 is supplied by a DC. source 16 which is connected to awattmeter 17 which supplies a signal P proportional to the DC. energy Psupplied by the source 17, that is to say, absorbed by the amplifier 13.A calculating member of the adder relay type 18 receives at its threeinputs the signal P P P and supplies a signal P equal to P P +P that is,according to Equation 3 above, varying with R, the energy dissipated inthe amplifier 13. This member 18 applies the signal P,, to the input ofa threshold comparator 19 which receives at its other input a referencevalue W.

The member 19 can supply under certain conditions an output signal ywhich, applied to the reducer 12, reduces the excitation of theamplifier 13. The member 19 supplies a reduction signal y when thesignal P exceeds the reference signal W. In other words, if the energydissipated in the amplifier, namely P exceeds a predeter-' minedreference value W, the excitation level at the input of the amplifier 13is reduced sufficiently so that the energy P assumes a new value whichis lower than, or equal to, W.

FIGURE 2 shows diagrammatically the application of the invention to atransmitter comprising a pilot oscillator 21, a modulator 22, receivingat one input the output current of the oscillator and at the otherinput, for example, the microphonic current supplied by a microphone2201. The modulated high-frequency current is amplified by a variablegain pre-amplifier 23, which excites a power amplifier 24 (Class AB, orClass B, or Class C) whose output circuit is matched to an antenna 27 bya tuning and matching circuit 25. Between the tuning and matchingcircuit 25 and the antenna 27 is a reflectometer 26 which supplies anoutput signal P at one output, and a signal P at another output, where Pand P are as defined above.

The amplifier 24 is supplied with constant voltage via two terminalsmarked and respectively, by a DC. source 30, whose negative terminal 31is connected to ground through a resistor R of relatively low value, andthe positive terminal 32 of the source 30 is connected to the terminalof the amplifier 24. The voltage drop V across this resistor isproportional to the current flowing therethrough, that is, to the energyP consumed by the amplifier 24.

A first differential amplifier 28 receives at its two in- .4 puts thesignals P and P and supplies at the output a signal P equal to thedifference P P This signal P varies as the energy effectivelytransmitted to the load P according to Equation 1 above.

A second differential amplifier 29 receives at one input the signal P,and at another input the signal V, which is equivalent to the signal P:V=P This second differential amplifier supplies an output voltage Uwhich is equivalent to the signal P mentioned above.

A battery 40 with the electromotive force U has its positive terminal 41connected to ground and its negative terminal 42 connected to a gaincontrol terminal 23a of the preamplifier 23 across a resistor R Avoltage U is applied to the terminal 23a. Between the terminal 23a andthe output of the amplifier 29 is a diode 50 whose cathode returns toground through the output circuit of the amplifier 29. The voltage Uobtained from the output of the amplifier 29 is equivalent to the signalP of FIGURE 1.

For an absolute value of the voltage U (negative) below a certainthreshold, the modulus of U is lower than the modulus of U and the diode50 is therefore polarized in reverse, that is, it is blocked, and thevoltage U is equal to U because it does not pass current to the resistorR For a modulus of U exceeding a certain fixed value (i.e., for a valueof the dissipated energy P exceeding a predetermined threshold), [Ubecomes greater than [U and the diode 50 becomes conducting; the voltageU is therefore aligned with U under the action of a current I passingthrough the resistor R The voltage U therefore becomes more negative;and from this follows a drop in the gain of the preamplifier 23, andthus a reduction of the excitation of the power amplifier and areduction in the energy dissipated in this amplifier. The transmissioncontinues at reduced level and the desired result is thus obtained.

For demonstrating the advantages achieved by the invention over andabove known devices, the following table shows the values of differentparameters obtained from a transmitter during operation. The standingwave ratio S is equal to 1+r/1r, where r= /P /1 the gain of thepreamplifier is designated C, and the dissipated energy not to beexceeded has been fixed at W=l500 watts. Four operating conditions areshown, namely (a), (b), (c) and (d). The power is expressed in watts,the voltage in volts, and the gain in decibels.

S P0 P P2 P P4 U1 U2 0 Condition (a) is normal operation. The antenna ismatched, and the standing wave ratio S is unity. The diode 50 isreversely polarized. The source voltage (U =U =6v.) is applied to thepreamplifier 23 which has in this case a gain of 20 db.

In condition (h) the load impedance rises. It follows therefore, on theone hand, that there occurs a mismatching between the amplifier and theload which appears as a reduction of the power P taken up by the load,passing from 1000 to 450 w., and causes a standing wave ratio S of 3,and on the hand, a reduction in the energy taken from the DC. source,namely P owing to the curvature of the characteristic of the powertubes, so that the peak current drops while the load impedance rises.The output voltage of the differential amplifier 29 assumes the value U=3 v.; thus the diode remains blocked, and the gain of the preamplifier23 remains at 20 db.

For condition (c) it will be assumed that the diode 50 is disconnectedfrom the amplifier 29, thereby suppressing the power safety control. Inthe present case, the standing wave ratio S has the same value as incase (b), but the load impedance has been reduced. It follows therefrom,owing to the curvature of the power tube characteristic causing anincrease in the peak current, that there occurs a rise in the power Pwith a dissipated power P assuming the value of 1740 watts, which isexcessive and dangerous. At the same time, U assumes the value -9.5 v.;but since the control loop is open, no correction is made in the gain ofthe amplifier, which remains at 20 db.

For condition (d) the impedance conditions of the load are the same asunder (c), but the loop is closed. In the state of equilibrium, theoutput voltage of the differential amplifier 29, U is equal to 7.7 v.Since the diode 50 is open, the anode side carries a voltage U whichdiffers from U only by the value of the elbow voltage, equal to 0.7 v.For a voltage U equal to 7 v. the gain of the preamplifier 23 is broughtto 19 db.

An arrangement of known construction, acting as a function of thestanding wave ratio, would trigger off in the case (b), where the tubesare in no danger whatever with a dissipation of 550 w. However, thedevice according to the invention, does not respond in this instance.And yet, the device according to the invention does automaticallyprotect the installation in the case (d) while assuring the continuationof the transmission, with a power to the antenna only reduced to 293watt.

The control of the power absorbed by the amplifier by the action on thealternative excitation level is generally advantageous, but othermethods of controlling are possible, without thereby departing from theprinciple of the invention. For example, a control of the screengridvoltage of a pentode or of a tetrode, is also possible.

It is also possible to carry out the calculation by a numerical channel,using a numerical unit and analoguenumerical converters. Such aninstallation may be advantageous in the case where several amplifiersare centrally controlled.

'We claim:

1. A system for automatically controlling the energy dissipation in anamplifier receiving energy from a D.C. source by maintaining said energydissipation below a maximum value, comprising:

an amplifier connected between a signal source and a load, and a D.C.source connected to said amplifier for providing energizing potentialthereto,

first means connected between said amplifier and said load for measuringthe energy supplied by the amplifier to said load and the energyreflected from said load back to said amplifier,

second means connected to said D.C. source for measuring the energysupplied by said source to said amplifier,

third means connected to said first and said second means forcalculating the energy dissipation of said amplifier from themeasurements of said first and second means, and

fourth means connected to the output of said third means for reducingthe signal level of the input to said amplifier from said signal sourcein-response to detection of an energy dissipation in said amplifierabove a maximum value.

2. The combination as defined in claim 1 wherein said first meansconsists of a reflectometer providing a first output P proportional tothe energy supplied by the amplifier to said load and a second output Pproportional to the energy reflected from said load back to saidamplifier.

3. The combination as defined in claim 2 wherein said second meansconsists of a wattmeter circuit providing an output P proportional tothe energy supplied by said source to said amplifier.

4. The combination as defined in claim 3 wherein said third means is acalculating device providing an output P =P P +P proportional to theenergy dissipation of said amplifier.

5. The combination as defined in claim 4 wherein said fourth meansincludes a threshold comparator connected to said calculating device andproviding an output control signal in response to said output Pexceeding a threshold value, and a control member connected between saidsignal source and said amplifier and responsive to said control signalfor reducing the excitation level of said amplifier.

6. The combination defined in claim 5 wherein said control member actson the alternative excitation level of the power amplifier.

7. The combination as defined in claim 5 wherein said control memberincludes a preamplifier whose gain is varied with variation in D.C.voltage supply thereto.

8. The combination as defined in claim 3 wherein said wattmeter circuitis formed by a resistor of low value connected in the ground connectionof said D.C. source.

9. The combination as defined in claim 4 wherein said caluclating devicecomprises two differential amplifiers connected in cascade, the firstdifferential amplifier providing an output P =P P and the seconddifferential amplifier providing an output P =P P 10. The combination asdefined in claim 7 wherein said threshold comparator comprises a diodeconnected between the output of said calculating device and a D.C.supply connected to said preamplifier, said diode providing control ofsaid preamplifier when said output P exceed the value of said D.C.supply.

References Cited UNITED STATES PATENTS 2,649,570 8/1953 Radclifi'e 330-2X NATHAN KAUFMAN, Primary Examiner US. Cl. X.R. 330-144

